Molecular Mechanism for the (-)-Epigallocatechin Gallate-Induced Toxic to Nontoxic Remodeling of Aβ Oligomers

(-)-Epigallocatechin gallate (EGCG) effectively reduces the cytotoxicity of the Alzheimer's disease beta-amyloid peptide (A beta) by remodeling seeding-competent A beta oligomers into off-pathway seeding-incompetent A beta assemblies. However, the mechanism of EGCG-induced remodeling is not fully understood. Here we combine 15N and 1H dark-state exchange saturation transfer (DEST), relaxation, and chemical shift projection NMR analyses with fluorescence, dynamic light scattering, and electron microscopy to elucidate how EGCG remodels A beta oligomers. We show that the remodeling adheres to a Hill-Scatchard model whereby the A beta(1-40) self-association occurs cooperatively and generates A beta(1-40) oligomers with multiple independent binding sites for EGCG with a Kd similar to 10-fold lower than that for the A beta(1-40) monomers. Upon binding to EGCG, the A beta(1-40) oligomers become less solvent exposed, and the beta-regions, which are involved in direct monomer-protofibril contacts in the absence of EGCG, undergo a direct-to-tethered contact shift. This switch toward less engaged monomer-protofibril contacts explains the seeding incompetency observed upon EGCG remodeling and suggests that EGCG interferes with secondary nucleation events known to generate toxic A beta assemblies. Unexpectedly, the N-terminal residues experience an opposite EGCG-induced shift from tethered to direct contacts, explaining why EGCG remodeling occurs without release of A beta(1-40) monomers. We also show that upon binding A beta(1-40) oligomers the relative positions of the EGCG B and D rings change with respect to that of ring A. These distinct structural changes occurring in both A beta(1-40) oligomers and EGCG during remodeling offer a foundation for understanding the molecular mechanism of EGCG as a neurotoxicity inhibitor. Furthermore, the results reported here illustrate the effectiveness of DEST-based NMR approaches in investigating the mechanism of low-molecular-weight amyloid inhibitors.